1. Field of the Invention
This invention relates to sealing and packing systems used in subterranean oil and gas wells for providing an annular seal between a fluid transmission conduit disposed in another conduit, the most common example being to provide an annular seal between production tubing and casing.
2. Description of the Prior Art
There are many different downhole tools in the oil and gas industry which require that a seal be established in the annulus between a fluid transmission conduit or tubing string disposed in a well bore and the outer well casing. These tools may relate to the drilling and completion of the well, the production of the well, the servicing of the well, or the abandonment of the well. Conventional packers, employing an anchoring system for holding a sealing element in position against either upwardly or downwardly acting pressure differentials, are most often employed for establishing this seal. These conventional packers generally employ radially expandable anchor slip systems and radially expandable packing or sealing elements to prevent fluid communication and to provide pressure integrity. Such packers are typically run in and set in place either by or on a tubing string or a wireline setting tool. When set using a tubing string, the packer is typically set using hydraulic pressure in the tubing, hydrostatic pressure in the well bore, or a combination of both. It may also be mechanically set by the weight of the tubing. These packers can be permanent type packers with an internal seal bore for receiving tubing which can be retrieved while leaving the packer in place. Retrievable packers, employing techniques such as rotary manipulation of the tubing string to release anchor slip assemblies and packing elements for retrieval of the packing element, are also commonly employed.
It is generally necessary that sealing integrity be established between separate elements within the tubing string or between accessory items and the tubing string. For example, it is generally necessary that a tubing section, inserted into a seal bore of a packer, must have sealing integrity between that section and the packer. One means of providing such sealing integrity is to utilize stacks of sealing elements in which individual sealing elements have a generally chevron-shaped cross section. Sealing systems employing such chevron-shaped sealing elements are depicted on page 672 of the 1980-1981 Composite Catalog of Oilfield Equipment and Services published by World Oil. These chevron-shaped sealing elements and systems, commonly referred to as tubing seal systems, are generally employed to establish a seal between a tubing mounted element and the internal seal bore of a conventional packer. An alternative method of establishing a seal between a conventional packer and tubing elements while still permitting movement of the tubing elements relative to the packer, is depicted in U.S. Pat. No. 3,109,490 covering a slidable latching seal assembly.
In addition to the use of conventional packing elements to provide sealing integrity in the tubing casing annulus and to isolate the production zone from portions of the annulus extending above the packing element, casing polished bore receptacles have been employed in conjunction with sealing elements to achieve some of the objectives achieved by conventional packers. A typical prior art example of the use of packoff assemblies in conjunction with casing bore receptacles, or liners, is discussed on pages 6438 and 6439 of the 1978-79 Composite Catalog of Oilfield Equipment and Services published by World Oil.
In copending application Ser. No. 273,805, filed June 16, 1981, and assigned to the assignee of the instant invention, there is disclosed and claimed a packoff assembly which can be used in conjunction with a casing bore receptacle and a tubing mounted mandrel to provide a tubing-casing annular seal and to permit isolation of the production zone from the tubing-casing annulus. Such packoff assembly can be positioned at a precise location in the casing and will permit tubing movement which may result during a production or treating cycle.
Tubing movement is especially significant in deep hot wells. In deep hot wells, the tubing is originally landed at more or less than ambient well temperature. During treating operations, for example, if a cold acid is pumped down the tubing, the tubing would tend to undergo contraction. The tubing will elongate if heated by produced fluids. Some means for permitting tubing movement must therefore be provided. Tubing may tend to shrink because of a ballooning effect or as a result of helical buckling. The tubing may also be subjected to a compressive force, sometimes referred to as "piston effect", tending to shorten the tubing. This force is due to differential pressure acting on the end area of the tubing and that portion of a packoff assembly extending between the tubing and casing. The invention of the aforementioned copending application provides a means for attaching the tubing casing packoff assembly to the casing, thus eliminating any piston effect. By attaching the packoff assembly directly to the casing receptacle, the only force acting on the tubing would be that force developed by the pressure differential acting on the cross-sectional area of the tubing itself.
The structure disclosed in said copending application also permits the use of tubing seal systems to accomplish the sealing function otherwise achieved by the use of conventional radially expanding packing elements. With such construction, the cross-sectional area or gap across which the sealing elements must bridge is much less than that encountered when conventional packing elements are used. Significant radial expansion of the sealing elements themselves is therefore eliminated.
Conventional radially expanding packing elements generally required a complex means of expanding the packing element into and maintaining it in sealing engagement with the surface to be sealed. This means is sometimes further complicated by the necessity of providing expanding packing element retaining means to prevent extrusion of the packing element through the gap that it must bridge. On the other hand, the elements of tubing seal systems are energized by the pressure which they contain. They therefore need no mechanism to expand them and since the metal elements that retain them fit the sealing surface closely, there can be a very small gap that the seals must bridge. Contraction of the packoff assembly using a tubing seal system is, therefore, much more simple than one using conventional radially expanding packing elements.
The specific apparatus disclosed in the aforementioned copending application relied upon the expansion of collet arms to effect the securement of the packoff assembly to the casing receptacle. Such collet arms were forced outwardly into their latching position by a relative downward movement of the polished external surface of the mandrel which also cooperated in sealing relationship with internal seals of the packoff assembly. As the mandrel moved up and down in response to expansion or contraction movements of the tubing string, there was continuous frictional contact between the collet arms and the polished mandrel surface, thus creating the possibility of scratching and in any event producing a brinelling or localized hardening of the polished seal surface which adversely affected its cooperation with the internal seals of the packoff assembly.
Additionally, the packoff assembly of the aforementioned copending application was removed from engagement with the casing receptacle by upward movement of the tubing string and interconnected mandrel, which upward force was transmitted to the packoff assembly through the collet arms. In the event that the packoff assembly was wedged, jammed, or corroded in its installed position, there was the possibility of buckling the collet arms in the effort to effect the removal of the packoff assembly. Hence, further improvements in a packoff assembly utilizing an annular packoff member cooperable with a casing bore receptacle are desirable.